I personally would want to be convinced that a ribbon UI has any advantage at all...
Besides, it's an almost Windows-only thing. Mac and Linux users would be pretty hard to convince to embrace such a change too...

I need as much space as possible for the 3D view / Sketcher. If ribbons can help to save space at the top - welcome. But remembering the Windows ribbons where the text contents of a document seems to become less and less important, I probably won't like it.

Actually, I think my idea is better understood in terms of browser tabs... Imagine the menu bar and workbenches converted to browser tabs. Clicking on one exposes toolbars specific to that menu / workbench. Like browser tabs, new UI tabs can be added / removed. Adding a tab would open a dialog, allowing the user to select a workbench that they want to add to the UI. Some tabs might be pinned (like the "File", "Edit", "View" menus, etc.) so they can't be closed.

The idea is that you can customize the UI so that the workbenches you use most are always "open", so to speak - they have a persistent tab at the top of the UI. You can add / remove at your leisure and we could even create "workflows" which create UI layouts with specific workbenches for specific purposes (like Arch, CNC, Simulation, Animation, etc.)

I can't say it would necessarily make the UI more functional overall, but it seems as though it would make it more intuitive, possible reduce a little clutter, and possibly discourage a workbench developer from trying to modify the UI at large...

Here's why I like the idea: FreeCAD is a generic 3D parametric CAD tool. You create / import 3D data and you manipulate it by switching between workbenches to access specific tools. By having tabs at the top, it communicates that idea more intuitively. The user sees tabs marked "Part Design", "Tech Draw", "Animation", etc, and they intuitively understand that those benches are "open" - available for them to use. In the current scheme, you only see the work bench which is currently active - all the others are "closed' or, intuitively, unavailable.

Again, this doesn't change anything about how FC works, but it communicates more intutively how FC is intended to work. It "flows" better, in my mind, I guess. And I think it could useful in managing UI clutter a little bit.